Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Mechanical tap cha

A mechanical tap changer physically makes the new connection before releasing the old using multiple tap selector switches, but avoids creating high circulating currents by using a diverter switch to temporarily place a large diverter impedance in series with the
short-circuited turns. This technique overcomes the problems with open
or short circuit taps. In a resistance type tap changer, the changeover
must be made rapidly to avoid overheating of the diverter. A reactance
type tap changer uses a dedicated preventive autotransformer winding to function as the diverter impedance, and a reactance type tap
changer is usually designed to sustain off-tap loading indefinitely.

In a typical diverter switch powerful springs are tensioned by a low
power motor (motor drive unit (MDU)), and then rapidly released to
effect the tap changing operation. To reduce arcing at the contacts, the tap changer operates in a chamber filled with insulating transformer oil, or inside an SF6 vessel. Reactance-type tap changers, when operating in oil, must allow
for with the additional inductive flyback generated by the
autotransformer and commonly include a vacuum bottle in parallel with
the diverter switch. During a tap-change operation, the flyback raises
the potential between the two electrodes in the bottle, and some of the
energy is dissipated in an arc discharge through the bottle instead of
flashing across the diverter switch.

Some arcing is unavoidable, and both the tap changer oil and the
switch contacts will slowly deteriorate with use. To prevent
contamination of the tank oil and facilitate maintenance operations, the
diverter switch usually operates in a separate compartment from the
main transformer tank, and often the tap selector switches will be
located in the compartment as well. All of the winding taps will then be
routed into the tap changer compartment through a terminal array.

One possible design (flag type) of on-load mechanical tap changer is
shown to the right. It commences operation at tap position 2, with load
supplied directly via the right hand connection. Diverter resistor A is
short-circuited; diverter B is unused.